Recently, demand is increasing for high-performance lithium rechargeable batteries and the like for use in portable information terminals, portable electronic devices, compact power storage devices for home use, and electric motor bicycles, electric cars, hybrid electric cars and the like that are powered by electric motors. The rechargeable battery is a battery capable of being charged and discharged. During use, the rechargeable battery is subjected to repeated charge and discharge over a long period of time. Its components therefore have to have long-term stability and durability.
When the purity of a raw material such as a solid electrolyte used in a rechargeable battery is low, deterioration of the components is liable to be accelerated. Thus, purity of the solid electrolyte or other such raw material is required to be high.
The applicant found a method of producing lithium sulfide which can produce lithium sulfide in a high purity using simple means (JP-A-H07-330312). In this method, lithium sulfide is produced in an aprotic organic solvent such as N-methyl-2-pyrrolidone (hereinafter referred to as NMP), and it is an economical and simple method because the production steps thereof can be conducted continuously. However, LMAB, which is an impurity derived from NMP, comes to be mixed in the resultant lithium sulfide.
JP-A-H09-283156 discloses a method of producing lithium sulfide wherein lithium hydroxide and a gaseous sulfur source are reacted at a temperature of at lowest 130° C. to at highest 445° C. Sulfur oxides (such as lithium sulfite, lithium sulfate, and lithium thiosulfate) which are formed in the production process of lithium sulfide come to be mixed in the lithium sulfide produced by this method.
When the lithium sulfide produced by the above-mentioned methods is used as a raw material of a solid electrolyte for a lithium rechargeable battery or the like, the solid electrolyte deteriorates due to the repetition of charge and discharge, and the desired battery performance cannot be realized.
One object of the invention is to solve the above-mentioned problems by providing a method of purifying lithium sulfide wherein impurities contained in lithium sulfide constituting a raw material for a solid electrolyte of a lithium rechargeable battery are minimized.
Another object of the invention is to provide a solid electrolyte for a lithium rechargeable battery using lithium sulfide in which the impurities are minimal, and a solid battery using the same.